rocket OT

[RJ Guitars]

Cliff,

I was watching kinda hoping somebody informed would answer... in their absence I will make a guess or two.

it's my best guess that it will work and you will have a good amp. It's hard to say what the impact of the borderline rating means. The Trainwreck output transformers are usually 35 watts but if you've noticed they are really heafty units and more substantial than a 50 watt Fender Output tranny.

I myself am an experimenter and there aren't really bad results as long as they eventually lead to good sounding amps and smarter amp builders. I say do it and hear it and report back to us so we are a little bit wiser. I exppect that Ken Fischer had his reasons for going with the heavy iron, maybe you'll give us the technical reasons why he did it?

rj

[Zippy]

Would a 30W, 4400Ohm CT transformer be enough for a Rocket? Seems sort of against the KF philosophy to use underrated transformers (30W vs. 35W) but my concern isn't necessarily the power handling, it's the 70mA that the transformer can handle that sorta worries me.

How are you differentiating between power and current capability, Cliff? Given the same ouptput impedence, power is defined by the current.

Beyond that, my bet is that the extra iron keeps the OT further from saturation and yields more definition in the bass response. Extra power margin helps the OT run cooler as well as its dissipation will be greater.

[bancika]

the data sheet says that two el84 in PP it need 8k load and can draw up to 90mA. 4 tubes would be nice match with 4.4k primary but if I'm getting the math right, they would draw up to 180mA.

[FYL]

Your math is correct. A pair of EL84's in AB1 draw app. 70 mA plate current idling, 90 mA max - only one tube draws current above quiescent at any time.

A quad will draw 140 / 180 mA, so the OT primary windings need to be sized accordingly - wire gauge, etc.

For instance the Dynaco A-470 is rated at 80 mA per side, well below the required 140 /180 mA, but is listed as compatible with a quad of EL84's because it can handle twice the rated DC current (cf pix below, from a Dynaco catalog).

So no problemo here, except if the amp is maxxed for more than a few minutes - heavy stage use, cranked output to an attenuator, etc. - then the OT will probably self destruct.

Oh and the $26.95 retail price was correct when Richard Nixon fiddled with tape recorders in the Oval Office... Today's equivalent, the Dynaclone A-470S (manufactured by Magnetic Components) retails for $94.95.

[Cliff Schecht]

In a push pull quad set, only two of the four tubes are shoving DC current through the transformer at any given time. I calculate 290V and a 4400Ohm CT gives me about 33mA per tube.. Can you guys confirm this? If so then I'm fine!

[RJ Guitars]

That requires consideration of the old "is the AC-30 a class A or Class AB amp question"... or maybe the more appropriate question of "will your amp be operating as a class A or class AB amp?

Then maybe the added discussion of will you keep the amp on 11 for any extended time units?

rj

[FYL]

In a push pull quad set, only two of the four tubes are shoving DC current through the transformer at any given time. I calculate 290V and a 4400Ohm CT gives me about 33mA per tube.. Can you guys confirm this? If so then I'm fine!

Sorry you're not. Your calculation could be correct for a class B amp, with one half of the push-pull going into cut-off while the other is active. We're dealing with deep class AB, where idle - i.e. bias - current is substantial and where there is very little difference between the two halves of the PP.

[Cliff Schecht]

I wasn't taking into account the fact that while one side is pushing, the other side still will have some idle current to keep the tubes operating. Like you say FYL, the actual current draw is quite a bit higher in class AB and with a 50 Ohm cathode resistor, Ken is pushing those output tubes pretty hard. Having such a low B+ is what is preventing me from using the iron that I have on hand. Raising the B+ would allow me to use this transformer but this would change the sound of the amplifier as well, and not necessarily for the better. It would be better for me to use a proper transformer set when I get the money to do so.. :-/

[M Fowler]

Seems to me you already changed the sound of the Rocket when you were considering the tranny change from what Ken F used in the first place right?

So use what you have and it will make for a great amp.

[Cliff Schecht]

I don't think I can use it actually. This is why I was asking for the current that I should be expecting through the transformer. The ones I have specify 70mA max. A "real" Rocket is too far past this rating and I'm not so into trying to modify Ken's circuit to suit the transformer, I'd rather design something from scratch. Besides, if I wreck this iron (hehe) then I would have to buy a real transformer set anyways and I'd be out a cool old piece of iron..

These units were really designed for hifi PP 6L6 amps (they have a 10% inverse feedback winding!) and so I plan on eventually designing a hifi amp around them. I may use one of them in my upcoming Deluxe build but I'll keep the leads nice and long so it's reusable in the future.

[M Fowler]

The spec for a AC30 which the Rocket is identical on the bright channel except for no bypass cap on the 1k5 bias V1b is OT 4k primary 30w rating.

Mark